CCA-0.1.5.3: preprocessor/State.hs

-- A Haskell-98-compatible subset of the Control.Monad.State module.

module State(State, runState, get, put) where

import Control.Applicative
import Control.Monad (ap, liftM)

newtype State s a = State { runState :: s -> (a, s) }

instance Functor (State s) where
  fmap = liftM

instance Applicative (State s) where
  pure = return
  (<*>) = ap

instance Monad (State s) where
  return x = State (\n -> (x, n))
  State v >>= f = State (\n -> let (x, n') = v n in runState (f x) n')

get :: State s s
get = State (\s -> (s, s))

put :: s -> State s ()
put s = State (\_ -> ((), s))